Frankie Valli was photographed at the Starkey Hearing Foundation ball in 2015. (Bre McGee/Special to the Star Tribune)

As the fireworks exploded after Frankie Valli and the Four Seasons’ performance Monday night at the State Fair grandstand, a middle-aged woman approached a teenager in the audience and praised him for his consistent enthusiasm throughout the concert.

Earlier, the 17-year-old told me that Valli was his favorite singer. That has been the case since the youngster discovered the 1960s pop star three or four years ago in the musical “Jersey Boys” at the Orpheum Theatre in Minneapolis.  The kid proudly showed up in the fifth row at the fair clutching his LP of the group’s “Sherry.”

Hate to burst the kid’s bubble but, no matter how entertaining and generous Valli’s show was, it was too much lip-syncing.

Valli, 83, barely moved his lips. He rarely looked as if he was taking the kind of deep breaths required to sing with palpable passion. And he never seemed the least bit winded after any song.

Maybe I’m wrong and the Rock Hall of Famer is a living wonder, someone who can summon his falsetto at will and sing high notes so effortlessly he should be a coach on “American Idol.”

He is an American idol, his career rejuvenated by the smash success of “Jersey Boys.” He drew 9,320 folks to the fair to hear “Rag Doll,” “Walk Like a Man” and “Big Girls Don’t Cry.”

Backed by four younger Seasons who performed with verve and vitality, Valli played a rewarding 100 minutes. The show was slicker than a PJ Fleck pep talk especially the way the four seasoned backup singers, notably Brandon Brigham, were covering (or bolstering) Valli’s part when it wasn’t plain old lip syncing. When Valli did sing live, his voice sounded a little rough in his mid-range.

In addition to doing hits and obscurities (“Beggin’,” “Save It for Me”) from the Four Seasons catalog, Valli and the new Seasons offered their versions of some old ‘60s hits, including “Spanish Harlem,” a cool mashup of the Temptations’ “My Girl” and the Rascals’ “Groovin’” as well as “Stay,” with lots of faux falsetto from Frankie.

Faux because I think it was lip syncing. Or maybe Frankie fooled me and not the thrilled 17-year-old kid seeing him for the first time.